Solid oxide electrolyte fuel cells ("SOFC") of the tubular variety have been under investigation for over 20 years, are well known in the art, and are taught, for example, in U.S. Pat. No. 4,490,444 (Isenberg). The SOFC tube generally contains an inner, porous, self supporting "air" electrode in contact with oxidant in operation, upon which are disposed a dense electrolyte, usually of yttria stabilized zirconia, and an outer "fuel" electrode in contact with a fuel in operation. In some instances, the air electrode can have a separate, porous, inner support.
In producing these SOFC tubes, sophisticated chemical/electrochemical vapor deposition techniques ("CVD/EVD") are used to deposit the electrolyte and fuel electrode, as taught in U.S. Pat. Nos. 4,609,562 (Isenberg et al.) and 4,597,170 (Isenberg) respectively. The apparatus used for these depositions includes means for controlling the pressure and flow rate of gaseous reactants on both sides of the porous air electrode, a manometer for measuring the difference in pressure between the gaseous reactants on each side of the electrode, and means for changing the difference in pressure between the gaseous reactants. The vapor used to form the yttrium stabilized zirconia will contain a specific percentage of yttrium and zirconium chloride, which metal halide, upon contact with oxygen as another gaseous reactant will form this appropriate electrolyte metal oxide coating on the air electrode. As the metal oxide reaction product grows on the substrate, it closes off the pores in the substrate by chemical vapor deposition (CVD). The coating then continues to grow by electrochemical vapor deposition (EVD), because oxygen as ions move from the source of oxygen gas through the growing, dense electrolyte film.
In some instances an additional film of metal oxides selected from Mg, Ca+Al, Sr+Al, Zr, Y, Ce and their mixtures is formed on the outside fuel electrode, to prevent carbon formation, as taught in U.S. Pat. No. 4,894,297 (Singh et al.)
Also, a porous, discontinuous coating of cerium oxide(CeO.sub.2), in discrete particle form can be deposited at the air electrode interface with the electrolyte, as taught in U.S. Pat. No. 5,106,706 (Singh et al.) This particulate interface is used to help prevent partial air electrode encapsulation at the interface after SOFC operation at 1,000.degree. C., and to inhibit oxygen reduction reactions and reduce oxygen loss from air electrode particles during SOFC operation. These cerium oxide particles are deposited in a separate vacuum infiltration step, by precipitation from a hydroxide solution, and deposit in the air electrode pores. The particles are from 0.01 micrometer to 0.1 micrometer diameter, and are smaller than the sintered air electrode particles for ease of impregnation.
It has been found that during CVD formation of the electrolyte through the cerium oxide (CeO.sub.2) coating, the oxidant that flows through the pores of the air electrode tube may cause some CeO.sub.2 particulates to be displaced from the air electrode pores and surface, thus reducing the number of particles and the effectiveness of the layer. Additionally, metal halide vapors and by-products of the reaction, namely chlorine or hydrochloric acid can attack the discrete CeO.sub.2 particles, converting a portion of them to CeCl.sub.3 vapor and thus transport them out of the system. Both of these phenomena may be aggravated because of the high surface area CeO.sub.2 particles are not firmly affixed to the air electrode tube surface. As a result, not as much CeO.sub.2 may be on the air electrode surface as may be desired. Additionally, since CeO.sub.2 resides in the pores, it may not protect the entire electrochemical surface area and therefore may not produce the maximum performance/lifetime enhancement.
It is also known, that even with the current CeO.sub.2 particulate coating, after long-term electrical testing, the air electrode has been found to show structural changes in terms of porosity formation and densification. This is accelerated by high current operation. Such undesirable structural changes in the air electrode near the air electrode/electrolyte interface are postulated to be due to changes in the oxygen stoichiometry of the air electrode. This is thought to be aggravated by the `point contact` nature of the air electrode/particulate CeO.sub.2 electrolyte interface. Accelerated life tests at 1200.degree. C. have also shown degradation in performance with little increase in vacuum leak rate. It has been postulated that this degradation is due to the formation of undesirable insulating interfacial layers, such as La.sub.2 Zr.sub.2 O.sub.7. This formation may also be aggravated by point contact CeO.sub.2 sites.
Ideally, it would be beneficial to have CeO.sub.2 prevent intimate contact between the air electrode and the electrolyte. Cells with this type of microstructure could potentially exhibit better performance and lifetime characteristics due to a more uniformly distributed current density as well as improved protection of the interface.
Therefore, it is one of the main objects of this invention to reduce oxygen loss from the air electrode particles that are in contact with the electrolyte, increase the active area for the electron exchange reactions with oxygen at the electrode/electrolyte interface, to prevent formation of insulating interfacial layers, and to isolate the air electrode from the electrolyte. It would also be highly advantageous, if possible, to reduce the number of steps in the SOFC production process to reduce cost and make the SOFC more commercially viable.
In accordance with this objective, the present invention resides in an electrochemical cell comprising a porous fuel electrode and a porous air electrode, with solid oxide electrolyte therebetween, characterized in that the air electrode surface opposing the electrolyte has a separate, attached, dense continuous layer of a material comprising cerium oxide, and where electrolyte contacts the continuous cerium oxide layer, without contacting the air electrode.
The invention also resides in a chemical and electrochemical vapor deposition process for forming ceramic coatings on a porous ceramic air electrode substrate, where one of the coatings comprises a dense, continuous layer comprising cerium oxide, comprising the steps of: providing a first gaseous reactant comprising of oxygen on one side of said substrate which permeates through the pores in said substrate and reacts with a second gaseous reactant comprising cerium halide, that is provided on the other side of said substrate; passing additional of the first gaseous reactant to also directly contact the second gaseous reactant; and closing the pores by forming a dense, continuous layer comprising cerium oxide on the other side of the substrate.
An additional electrolyte layer can then be formed on top of the cerium oxide layer using appropriate electrolyte forming precursor gaseous halides and passing the first gaseous reactant only through the substrate followed by forming a final layer of a porous cermet (metal-ceramic) fuel electrode.
The continuous layer of cerium oxide is in solid film form, from approximately 0.001 micrometer to 5.0 micrometers thick. The oxide interface coating is from 90% to 100% of theoretical density ,and is essentially continuous and integral with the electrolyte layer and tightly bonded to the air electrode. The preferred electrochemical cell is a tubular fuel cell.
The continuous layer of, for example, dense cerium oxide, provides a more effective means to prevent encapsulation of or substantial penetration into the porous air electrode structure by the electrolyte material than prior art methods. Improved voltage--current and power--current characteristics are also shown as well as decreased sensitivity of electrical resistance vs. temperature, which may allow wider temperature cell operation. Also, very importantly, a separate infiltration step is eliminated and the interlayer is formed as part of the vapor deposition of the electrolyte, which can cut production costs very substantially. While the invention is described with regard to tubular fuel cells, if appropriate deposition apparatus were to be designed, the invention would also be useful for flat plate and other fuel cell configurations.